Active pixel having reduced dark current in a CMOS image sensor

ABSTRACT

The active pixel includes a photodiode, a transfer gate, and a reset transistor. The photodiode is substantially covered with an overlying structure, thus protecting the entire surface of the photodiode from damage. This substantially eliminates potential leakage current sources, which result in dark current. In one embodiment, the photodiode is covered by a FOX region in combination with the transfer gate.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of U.S. patentapplication Ser. No. 10/011,589, filed on Nov. 6, 2001, now U.S. Pat.No. 6,462,365, which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present invention relates to image sensing devices, and moreparticularly, to a pixel cell having reduced dark current.

BACKGROUND

CMOS image sensors have become the dominant solid state imagingtechnology, due in large part to their lower cost relative to CCDimaging devices. Further, for certain applications, CMOS devices aresuperior in performance. The pixel elements in a MOS device can be madesmaller and therefore provide a higher resolution than CCD imagesensors. In addition, signal processing logic can be integratedalongside the imaging circuitry, thus allowing for a single integratedchip to form a complete stand alone imaging device.

An active pixel sensor refers to an electronic image sensor with activedevices, such as transistors, within each pixel. Conventional activepixel sensors typically employ photodiodes as the image sensingelements. The most popular active pixel sensor structure consists ofthree transistors and an N+/P-well photodiode, which is a structure thatis compatible with the standard CMOS fabrication process. However, thisdesign has the drawback of a relatively large dark current (i.e., thecurrent that is output from the pixel in a dark environment).

It is desirable for the active pixel to have a low dark current.Excessive dark current lowers the dynamic range of the CMOS image sensorbecause there is insufficient ability to distinguish between light anddark conditions. In addition, dark current is the cause of the “whitepixel” defect in CMOS image sensors, i.e., a pixel that always outputs alarge signal.

Another active pixel sensor design that is not fabricated using thestandard CMOS fabrication process is the pinned photodiode, as shown inU.S. Pat. No. 5,625,210. The pinned photodiode has gained favor for itsability to have good color response for blue light, as well asadvantages in dark current density. Reduction in dark current isaccomplished by pinning the diode surface potential to the P-well orP-substrate (GND) through a P+ region. Because of the pinning P+ region,a transfer gate is necessary to output the charge of the photodiode to aN+ output region. An improvement to the '210 patent is shown in U.S.Pat. No. 5,880,495, assigned to the assignee of the present invention.

Nevertheless, the pinned photodiode configuration of the '210 still hasdark current effects. Further, the fabrication process for such aconfiguration requires significant modification form the standard CMOSfabrication prices, due to the buried channel transistor. The pinnedphotodiode configuration may cause image lag due to the incompletetransfer of charge from the diode to the floating node, if the junctionprofile is not perfectly optimized for the charge transfer.

Another approach in the context of CCD image sensors is to use ahydrogen anneal process to reduce dark current by passivating danglingsilicon bonds. For example, U.S. Pat. No. 6,271,054 discloses using sucha method. However, subsequent thermal processes, due to the poor thermalstability of the silicon-hydrogen structure, may easily destroy theeffect of hydrogen passivation.

Still another approach, disclosed in our co-pending patent applicationfiled Nov. 2, 2001 entitled “SURFACE PASSIVATION TO REDUCE DARK CURRENTIN A CMOS IMAGE SENSOR” to Wu et al., assigned to the assignee of thepresent invention, and incorporated by reference, teaches the use ofnitrogen, silicon, hydrogen, or oxygen to passivate the dangling siliconbonds in a CMOS compatible process.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of theinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a prior art active pixel.

FIGS. 2 and 3 are additional prior art active pixels.

FIGS. 4 and 5 are active pixels formed in accordance with the presentinvention.

FIG. 6 is an active pixel formed in accordance with an alternativeembodiment of the present invention.

FIGS. 7 and 8 are schematic diagrams of the active pixel formed inaccordance with the present invention.

FIG. 9 shows a top layout view of the active pixel of the presentinvention.

FIG. 10 shows an image sensor formed using the active pixels of thepresent invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are provided,such as the identification of various system components, to provide athorough understanding of embodiments of the invention. One skilled inthe art will recognize, however, that the invention can be practicedwithout one or more of the specific details, or with other methods,components, materials, etc. In still other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of various embodiments of theinvention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

The present invention is an active pixel sensor that can be formed withthe standard CMOS fabrication process, while also having the desirablecharacteristics of a low dark current. The active pixel sensor includesa plurality of active pixels arranged in a two-dimensional array. Theactive pixels have substantially the entire surface of the photodiodecovered either by a gate structure or a field oxide (FOX), therebyminimizing the amount of surface damage to the photodiode.

Turning first to FIG. 1, a prior art conventional active pixel 101 isshown. The active pixel 101 includes a photodiode having a P-well 105and a heavily doped N+ region 103 forming a p-n junction. The p-njunction is surrounded by an insulating oxide region, typically a FOXformed using a local oxidation of silicon (LOCOS) technique.Alternatively, the FOX may be a shallow trench isolation (STI).

The photodiode operates based on the principle of reverse-biasing a p-njunction diode such that a depletion region is formed. Next, thephotodiode is subjected to incident light, which travels throughtransparent oxide layers and into the silicon. The properties of thesemiconductor are such that electron-hole pairs are generated bothinside and outside the depletion region in response to the incidentphotons of visible light. The photon generated electron-hole pairs arethen swept away by diffusion and drift mechanisms and collected in thedepletion region, thereby inducing a photocurrent representing a portion(one “pixel”) of the desired image.

The current generated by the photodiode is used to modulate a pixeloutput transistor in source-follower configuration, as is conventionalin the prior art. Further, note that a field effect transistor is formedby the N+ region 103, a reset gate 107, and a second N+ region 109. Thereset gate 107 is conventionally formed from a stack of gate oxide andpolysilicon. The second N+ region 109 is typically tied to a highvoltage, such as V_(REF). When the reset gate 107 is turned on, thephotodiode is reset to the reset voltage (V_(REF)), as is alsoconvention in the prior art. Unlike a pinned photodiode pixel, notransfer gate is used in this prior art pixel design.

As noted previously, it has been found that the active pixel 101 of FIG.1 is susceptible to dark current, which may manifest as white pixels inthe image. One source of dark current is due to surface damage to thephotodiode. The surface damage is in the form of dangling silicon bondsand is often caused by manufacturing process related defects, includinggate and spacer etching stress. Another source of dark current comesfrom the FOX edge, which may result in mechanical stress or concentratedelectrical field at the edge.

A prior art attempt to reduce dark current is shown in FIGS. 2 and 3.Note that FIGS. 2 and 3 are substantially identical except that FIG. 2shows LOCOS isolation and FIG. 3 shows a STI. In this prior art, thephotodiode is formed beneath the isolation in the hopes of protectingthe photodiode from surface damage and minimize leakage current.However, the use of the N+ region 201 to connect to output circuitry isstill a potential leakage source susceptible to process induced damage.The active pixel of the present invention includes a transfer gate, aswell as a reset transistor. However, unlike the prior art, the entirephotodiode is substantially covered with an overlying structure, thusprotecting the entire surface of the photodiode from damage. Thissubstantially eliminates the potential leakage sources. In oneembodiment, the photodiode is covered by a FOX region in combinationwith the transfer gate. The invention shown in FIGS. 4-5 cansignificantly reduce surface damage, as well as relieve mechanicalstress at the FOX edge. As a result, the dark current can be reduced,thus, minimizing the number of white pixels.

Specifically, turning to FIGS. 4 and 5, an active pixel 401 formed inaccordance with the present invention is shown. FIGS. 4 and 5 aresubstantially similar, except that FIG. 4 shows a LOCOS isolation andFIG. 5 shows a STI structure. Further, FIG. 9 shows a top layout view ofthe active pixel 401 and FIGS. 7 and 8 show the active pixel 401 inschematic form. As will be further described below, FIG. 6 is analternative embodiment of the active pixel 401. FIGS. 4 and 5 are crosssection views taken along line 1-1′ of FIG. 9.

As seen in FIGS. 4-9, a transfer gate 108 is formed between an outputnode A (formed by an N+ region 110) and extending to at least the FOXregion. The transfer gate 108 is in one embodiment formed from a stackof gate oxide and polysilicon. However, it can be appreciated that thetransfer gate 108 may also be formed from other combinations ofmaterials or only one material. The transfer gate is controlled bysignal HD.

Further, the transfer gate 108 is sized and shaped to coversubstantially all of the photodiode 112 not already covered by the FOXregion. Note that in FIG. 9, the FOX region covers substantially all ofthe region, except for the diffusion (DIF). Like the active pixels shownin FIGS. 2 and 3, a large portion of the photodiode 112 is formedunderneath the FOX region. This minimizes the possibility of damage tothe surface of the photodiode 112.

However, unlike the prior art, an overlying structure is used to coverthe remainder of the photodiode 112. Thus, the transfer gate 108 isextended to cover the remainder of the active area (between the FOXregion and the N+ region 110). While the transfer gate 108 is shown toextend well over the FOX region, it can be appreciated that the extentof the overlap (if any) is variable and dependent upon processlimitations and design rules. Indeed, in one embodiment, the transfergate 108 extends to just the edge of the FOX region.

Note that in some embodiments, there may be a space between thephotodiode 112 and the P-well. This space is shown as dimension A inFIG. 4 and dimension B in FIG. 5. In other embodiments, the spacing maybe zero. In order to collect photo-generated charge, it is necessary toeffectively separate the photo-generated electron-hole (e-h) pairs tominimize recombination. Further, it is necessary to cause the carriersto reach a collection contact. Both of these objectives can be met usinga built-in electric field (such as a p-n junction) or by an externallyapplied field. It is desirable to have the e-h pair generation and theelectric field occur at the same location, so that generation andcollection can occur more effectively, e.g. increase the quantumefficiency (QE). This is the principle of operation of the presentpixel.

Note that the doping concentration in the P-sub region is significantlylower than the doping concentration in the P-well area. The gap (A or B)between the N-well 112 and the P-well can increase the depletion areaand that will increase the QE. However, the size of the gap depends onprocess and design rules, and in some cases, it may not be possible tohave a gap.

Node A also serves as the output of the photodiode 112. When thetransfer gate 108 is activated by the signal HD, the charge developed inthe photodiode 112 is shared with node A. This charge is then used tomodulate the output of a pixel output transistor (shown as M2 in FIGS. 7and 8) in source-follower configuration.

Further, in an alternative embodiment shown in FIG. 6, a surface N+region 201 may also be formed between the photodiode 112 and the P-well.The surface N+ region 201 is used to aid in the charge transfer from thephotodiode.

As best seen in the top layout view of FIG. 9, the active pixel 401 alsoincludes a reset transistor M1 (controlled by a RST signal), a rowselect transistor M3 (controlled by a RS signal), and the pixel outputtransistor (controlled by the signal on node A) M2. Note that in FIGS. 7and 8 the transfer gate 108 is denoted as a transistor like device M4controlled by signal HD.

FIG. 8 is an alternative embodiment that eliminates the row selecttransistor M3 (controlled by RS) because the existence of M4. In thisembodiment, the idea is to apply a different lower voltage (V_(REF-LOW))to the reset transistor M1 when the pixel is not selected (the RSTsignal should be high at that time so M1 in on). The voltage(V_(REF-LOW)) is low enough such that the output transistor M2 is off.In other words, M2 is used as a voltage buffer (source follower) whenthe pixel is selected and M2 is on at that time. M2 is off when thepixel is not selected and acts as a row select transistor.

Thus, the primary aspect of the present invention is that substantiallyall of the photodiode surface area is covered and protected. In oneembodiment, the photodiode 112 is covered by either the FOX region orthe transfer gate 108. Because of this, the amount of dark currentresulting from process induced surface damage or mechanical stress ofFOX edge is reduced significantly.

Note that the present invention teaches the use of a transfer gate 108(M4) between the photodiode 112 and the reset transistor M1. This allowsany dark current to be reset before readout, and as a result, darkcurrent can be further reduced.

Thus, output node A is reset before the signal charge is transferredfrom the photodiode. This can be accomplished either by applying avoltage constantly to node A (more suitable for the circuit in FIG. 8)or by applying a reset operation (by activating the reset transistor M1)before transfer of the signal charge (more suitable for the circuit inFIG. 7). The present invention “pushes” the major dark current to outputnode A, therefore, it is necessary to reset the dark current beforetransfer of the signal charge from the photodiode.

The active pixels described above may be used in a sensor array of aCMOS image sensor 1101. Specifically, FIG. 10 shows a CMOS image sensorformed in accordance with the present invention. The CMOS image sensorincludes a sensor array 1103, a processor circuit 1105, an input/output(I/O) 1107, memory 1109, and bus 1111. Preferably, each of thesecomponents is formed on a single silicon substrate and manufactured tobe integrated onto a single chip using standard CMOS processes.

The sensor array 1103 portion may be, for example, substantially similarto the sensor arrays portions of image sensors manufactured by theassignee of the present invention, OmniVision Technologies, Inc., ofSunnyvale, Calif., as model numbers OV7630, OV7920, OV7930, OV9620,OV9630, OV6910, or OV7640, except that the pixels are replaced with theactive pixels disclosed herein.

More specifically, the sensor array 1103 includes a plurality ofindividual pixels arranged in a two-dimensional array. In operation, asan image is focused onto the sensor array 1103, the sensor array 1103can obtain the raw image data.

The raw image data is then received by the processor circuit 1105 viabus 1111 to begin signal processing. The processor circuit 1105 iscapable of executing a set of preprogrammed instructions (perhaps storedin memory 1107) necessary to carry out the functions of the integratedcircuit 1101. The processor circuit 1105 may be a conventionalmicroprocessor, DSP, FPGA or a neuron circuit.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changed can be madetherein without departing from the spirit and scope of the invention. Itis also understood where the device has generally been shown usingdifferent types of P or N type materials, the types of materials couldbe switched to produce similar results. For example, rather than theN-well/P-sub photodiode that was formed, the alternate types ofmaterials could be used to form a P-well/N-sub photodiode. Thus,photodiodes that are N+/P-well, N+/P-sub, N-well/P-sub, P+/N-well,P-well/N-sub, etc . . . may also be used. Thus, the term PN photodiodeis defined to include all types of photodiodes. Further, an additionalimplant to adjust the threshold voltages (V_(t)) of the transfer orreset transistor may be performed to optimize charge transfer from thephotodiode.

Thus, the present invention has been described in relation to apreferred and several alternate embodiments. One of ordinary skill afterreading the foregoing specification will be able to affect variouschanges, alterations, and substitutions of equivalents without departingfrom the broad concepts disclosed. It is therefore intended that thescope of the letters patent granted hereon be limited only by thedefinitions contained in appended claims and equivalents thereof, andnot by limitations of the embodiments described herein.

We claim:
 1. An active pixel formed in a semiconductor substratecomprising: an isolation region formed on said substrate defining anactive area; a photodiode formed in said semiconductor substrate; anoutput node formed in said semiconductor substrate; and a transfer gateformed between said output node and said photodiode, such thatsubstantially the entire surface of the photodiode is covered by eitherthe transfer gate or said isolation region.
 2. The active pixel of claim1 further including a reset transistor formed within said active area,said reset transistor having a gate formed between said output node anda voltage reference (V_(REF)).
 3. The active pixel of claim 1 whereinsaid isolation region is a field oxide formed from local oxidation ofsilicon.
 4. The active pixel of claim 1 wherein said isolation region isa shallow trench isolation.
 5. The active pixel of claim 1 wherein saidsubstrate is a P-type substrate and said output node is a N+ region. 6.The active pixel of claim 1 wherein said output node is formed within aP-well formed in said P-type substrate.
 7. The active pixel of claim 6wherein said P-well is separated from said photodiode by a gap.
 8. Theactive pixel of claim 6 wherein said photodiode is formed by an N-wellformed in said P-type substrate.
 9. The active pixel of claim 1 furtherincluding a surface N+ region underneath said transfer gate andadjoining said photodiode.
 10. The active pixel of claim 1 furtherincluding a pixel output transistor formed within said active area andhaving its gate connected to said output node.
 11. A CMOS image sensorcomprising: a plurality of active pixels arranged in rows and columns,at least one of said active pixels comprising: (a) an isolation regionformed in a substrate defining an active area; (b) a light sensitiveelement formed in said semiconductor substrate; (c) an output nodeformed adjacent to said light sensitive element; and (d) a transfer gateformed between said output node and said photodiode, such thatsubstantially the entire surface of the light sensitive element iscovered by either the transfer gate or the isolation region; aprocessing circuit for receiving the output of said active pixels; andan I/O circuit for outputting the output of said active pixels off ofsaid CMOS image sensor.
 12. The image sensor of claim 11 further saidactive pixel further includes a reset transistor formed within saidactive area, said reset transistor having a gate formed between saidoutput node and a voltage reference (V_(REF)).
 13. The image sensor ofclaim 11 wherein said isolation region is formed from a field oxide. 14.The image sensor of claim 11 wherein said isolation region is formed bya shallow trench isolation.
 15. The image sensor of claim 11 whereinsaid substrate is a P-type substrate and said output node is a N+region.
 16. The image sensor of claim 11 wherein said active pixelfurther includes a surface N+ region underneath said transfer gate andadjoining said photodiode.